Water tanks, clarifiers and other similar water works tanks are constant targets of corrosion from the moment of installation. Effective and continuous steps must be taken to prevent corrosion or the result is costly replacement and repairs long before the expiration of the equipment's design life. Cathodic protection has become an important and accepted weapon in this battle against corrosion.
The corrosion of submerged metallic structures is caused by electrochemical activity. A metal surface in contact with an electrolyte, such as water, will normally contain both anodic and cathodic areas. In the cathodic areas, electric current flows from the electrolyte onto the metallic structure. The opposite electrical path is followed in anodic areas. The current flows from the metal to the electrolyte. Corrosion only occurs in anodic areas, where the current flows from, rather than to, the tank's interior surface. This particular electrical path sets the stage for the chemical formation of corrosion products. Cathodic protection prevents corrosion by assuring all areas of the metal's surface are cathodic, and none are anodic.
The impressed current method is the most common kind of cathodic protection used in water tanks. Direct current is sent through anodes suspended in the liquid, establishing a current flow from the anode, through the liquid, through the tank wall and to the ground. When the amount of current is adjusted properly, it overpowers corrosion current discharged from all anodic areas of the structure. The result is a net current flow onto all areas of the metal, the entire surface therefore being cathodic and safe from corrosive chemical activity.
To obtain the desired results the anodes must be mechanically suspended in the liquid to allow the electric current to flow through the water to the tank. In addition, the anodes must be arranged to achieve an even distribution of the cathodic protection current.
Winter weather is one of the worst enemies of a water tank cathodic protection system. Ice accumulates in the tank, often quickly and in quite comprehensive proportions in colder climates. In Canada and Alaska, for example, water will freeze one inch every twenty-four hours in a stagnant tank. Any exposed item is subject to damage due to direct accumulation. Additionally, in the correct conditions, the ice may rip away from the interior wall of the tank any attached fixtures such as support devices or eyelets for guys for cathodic protection systems.
Spring, and its warmer weather, may complicate rather than cure the ice accumulation conditions. With rising temperatures, the ice thaws and may break away from the side of the tank. When the ice drops, it may damage anything in its falling path, including cathodic protection systems.
Cathodic protection systems have been developed in which both the anodes and the supporting devices are submerged beneath water level. While submerged suspension systems may perhaps prevent the problems of direct accumulation and dropping ice, these systems are still susceptible to seasonal damage. The submerged systems may be of two basic designs, and in both designs wall supports are the only means of securing the system to the tank. In one design, the system is buoyant and secured to the tank by flexible wall supports or guys. In another design, the nonbuoyant submerged suspension system is secured to the tank by stable wall supports. When ice rips off the securing supports from the tank walls, the buoyant systems will float to the top hitting surface ice, while nonbuoyant systems will sink to the tank bottom. Therefore, despite the improvement made over roof-supported systems, the submerged suspension systems may still be victimized by ice damage in cold climates.
In addition to winter weather, the introduction of platinized niobium wire anodes has made conventional cathodic protection systems incompatible with current corrosion control needs. This anode material has a life span of 30 to 50 years, and is considered permanent because the wire will usually outlast the tank itself. Permanent anode wire is therefore more economic than traditional anodes which may require frequent replacement.
The advantages anode wire offers over other types of anodes has accounted for its increased use in protective systems. Along with the advantages, however, additional demands are placed on the corrosion control system, and some suspension systems of prior art are unsuitable. One added demand is that an anode wire suspension system's design must assure the anode material not come into contact with the tank during normal fluctuations in the water. The anode wire, being touch sensitive, also should not be allowed to contact surface ice in colder climates. Therefore, the prior art suspension systems susceptible to losing wall supports during the winter months are inadequate for long lived wire anode systems.
Another additional burden placed on the suspension system is that the anode wire should receive a connection from a current source every 10 amps or so many feet of wire. The present practice is to wrap feeder wire, which supplies the electrical current, around the anode wire. While this design enables the electrical connection to be made at required intervals, it has several disadvantages. The wrapping results in the feeder wire touching and covering a significant part of the anode wire's surface area. With each movement of water, a frequent occurrence in a tank in which the liquid level fluctuates often, platinum may be rubbed off the anode reducing the wire's effectiveness. Even in completely calm waters, the feeder wire is constantly shielding the current discharging from the anode. Furthermore, installation of the system may be complicated by the tendency of the two wires to become tangled with each other.
Examples of prior art buoyant systems referred to above may be seen in prior U.S. Pat. No. 3,718,554, and the PERMANODE.RTM. system marketed by Harco Technologies Corporation of Medina, Ohio, under such patent. The latter utilizes a single ring rope held in position by guys extending radially to the side wall of the tank, with buoys attached to the guys. If ice damage occurs to the guy anchors or to the electrical leads entering through the wall of the tank the system is apt to fail. Repair or replacement of the system may then require the tank to be emptied which can be costly as well as incovenient.